Physiology of Vision (2) بسم الله الرحمن الرحيم Physiology of Vision (2) Dr. Abdelrahman Mustafa Department Basic Medical Sciences Division of Physiology Faculty of Medicine Almaarefa Colleges

Learning Objectives By the end of this lecture, You should able to Contrast the transduction process for rods and the three types of cones, Describe the visual pathway Predict the visual field defects resulting from the following Describe the topographic representation of the visual field. Describe the processing of information in the visual cortex, and the consequence of a lesion in the higher visual association areas.

THE RODS AND CONES GRNETAL FEATURES These are photoreceptors of the retina Telereceptor Slowly to never adapt Contain specific photosensitive pigments which are chemically changed on exposure to light Such changes initiate action potential in the optic nerve fibers

Rods Cones 120 millions / retina 6 millions / retina At the periphery At Center (Fovea centralis) Rhodopsin pigment Photopsin (3types). More convergence (200 rods : 1 bipolar) One to one connection More sensitive to light (even to 1 photon). Less sensitive to light. Less accurate (less visual acuity). More accurate (Clear vision) More functioning at night (Scotopic vision) More functioning in day (Photobic vision) Vision in shades of grey Color perception (3 types for Red, Green & Blue)

Photopigments Undergo chemical alterations when activated by light Consists of 2 components: 1- Opsin:Protein that is integral part 2- Retinal:Derivative of vitamin ALight-Absorbing part of photopigment Rod pigment: Rhodopsin that Absorbs all visible wavelengths Cone pigments (color pigments):Respond selectively to various wavelengths of light. A single photon of light can activate a rod whereas several hundred photons are required to activate a cone

Photoreceptors

Visual Pigments: RODS Retinal + Opsin = Rhodopsin Functions only in dark, dim light & peripheral vision Regenerate only in dark or dim light situations The specific type of visual pigment used by rods is called Rhodopsin. * It is a combination of retinal plus the special opsin produced by rods. Rhodopsin is sometimes called visual purple. * While rhodopsin is sensitive to wavelengths of light throughout the visible light spectrum, it is most sensitive to green. * Since rhodopsin absorbs low intensity light, it functions only in the dark, in dim light situations and in peripheral vision. * When light reacts with rhodopsin, it causes the molecule to undergo a physical change in shape which causes the retinal and the changed opsin to physically separate. This separation causes the rod to send an impulse to the bipolar cell that may result in a nerve impulse by the ganglion cell. * Because of its sensitivity to light, activated opsin can only combine with retinal in the rod if little or light is available to trigger the reaction. To review the reaction ,* when rhodopsin is exposed to light, * * the light causes the opsin and retinal to separate, * which may generate a nerve impulse. * * (DIM Light) Impulse OPSIN RETINAL RHO DOPSIN

Visual Pigments: Cones Retinal + Red, Green or Blue Opsin = Red, Green or Blue visual pigments Function only in bright light (daylight) Provide sharp color images (Light) Visual pigments in cones react to light in a similar way, * except each visual pigment reacts to different wavelengths of light. * Each opsin absorbs light only in the area of the spectrum it is sensitive to: red, green or blue. * To stimulate a reaction in cones the light must be bright. Thus, cones only are stimulated in daylight situations or by light at night that is sufficiently bright. * Because of the arrangement of each cone synapsing with a single bipolar cell, the images are sharp and in shades of red, green or blue. * To review the reaction, a red cone when activated by light of sufficient intensity * will react, * causing the red opsin to separate from the retinal. * This will result in a nerve impulse to the brain in shades of red. * * If the wavelength of light is in the green part of the spectrum, * it will stimulate green cones to react * causing an impulse in shades of green to be sent to the brain. * Red Opsin RETINAL Impulse Red Cone Impulse Green Opsin RETINAL Green Cone

membrane hyperpolarizes DARK VERSUS LIGHT ACTION POTINAL CONDITIONS(TRASDUCTION) DARK rod cell Na+ channels open membrane depolarizes Na+ inflow stimulates glutamate release glutamate inhibits bipolar cells LIGHT rod cell Na+ channels close membrane hyperpolarizes no Na+ inflow prevents glutamate release bipolar cells initiate action potential visual pathway started NO signal in optic nerve

Dark versus light conditions

Color Blindness Causes: Most commonly hereditary (recessive sex - linked) and affecting males (8%) more than females (0.4 %). Very rarely acquired e.g. post encephalitis : 1-Trichromate: Patient has 3 cones but one of them is weak. 2Dichromate: One cone is totally absent & other two are present. 3- Monchromate: Patient has only one cone system and matches the different colors tested By Ishehara Chart

Visual Fields Each eye has two areas For vision NASAL FIELDS For front of vision Pathway through Temporal nerve CONTRA LATERALY TEMPORAL FIELDS For surrounding vision Pathway through NASAL nerves IPSILATERALY

visual pathway 1)Optic disc Collection of nerve fibers that transmit AP 2)Optic Nerve Ipislateral nasal nerve (same eye) Ipsilateral temporal nerve (same eye) 3)Optic chiasma Nasal nerves crosses to go to the opposite side of the brain DECASSATION 4)Optic Tract Ipislateral temporal nerve (same eye) Contaletral nasal nerve (other eye) 5) Lateral geniculonate Body The LGN contains a topographic representation of what the retina “sees”. This retinotopic map is sent to the visual cortex. 6)Optic Radiation From LGN to visual cortex

visual pathway Area 17 (=Primary visual area) 7)Primary visual cortex Area 17 (=Primary visual area) - Concerned with the appreciation of visual sensations. Area18(=Secondaryvisualarea/visual association area) - Concerned with correlation and integration of visual sensations. Area 19 (=Occipital eye field) - Concerned with movement of the eye.

Right eye point source of light Cornea Lens Iris Retina Vitreous When you look at something, like the sequence of letters on a printed page, the image is directed to the fovea and brought into sharp focus. The optical system of the eye works like a camera to focus an image on the surface of the retina. The cornea and the lens work together as the major refractive elements of the eye. The optical system of the eye creates a point to point topographic map of the visual world on the retina. The fovea is located about 3 mm temporal to the optic disc. The fovea is a critically important structure in the visual system as it subserves highest visual acuity in daylight, and has a profound impact on the organization of the rest of the visual pathway. Details about the neural and functional organization of the fovea will be considered in the next lecture. nasal temporal Optic nerve Fovea

Optic axons Ganglion cells Cone bipolar cells Here you see a cross section of the eye again with light passing through the optics and hitting the surface of the retina. The light sensitive elements, the photoreceptor cells, are actually closer to the back of the eye, so photons pass through several layers of retinal tissue, before being captured by the photoreceptors. In daylight, the signal generated in the photoreceptors then is transmitted via two synapses through a 3 neuron chain before a signal exits the eye via the optic nerve. That pathway is: cone photoreceptor to cone bipolar cell interneuron to ganglion cell. The axons of the ganglion cells collect on the retinal surface and bundle together passing out of the retina at the optic disc. In the next lecture we will consider in some detail the cell types and circuitry of the retina and how the initial signal generated by the photoreceptors is transformed by this circuitry. Cone photoreceptors

Optic nerve Optic tract Optic chiasm Hypo- thalamus Lateral geniculate nucleus Pretectum Superior colliculus Optic radiation The axons of the retinal ganglion cells pierce the sclera at the optic disc and form the optic nerve, cranial nerve II. At the ventral surface of the brain just anterior to the infindibulum the left and right optic nerves merge to form the optic chiasm. There is a partial decussation at the optic chiasm such that axons that originate in the nasal retina cross the midline and those that originate in the temporal retina remain uncrossed. Optic fibers terminate at multiple locations in the brainstem in both the diencephalon and the midbrain. Axons terminate in the hypothalamus and provide a light signal that plays a role in setting the 24 hr period of the circadian rhythms. Axons that terminate in the pretectum provide a signal that drives the pupillary light reflex (this pathway will be considered further in lab). At midbrain levels optic axons also terminate in the superior colliculus which plays an important role in the generation of precise saccadic eye movements to targets in visual space. As I said earlier the focus of these lectures is the primary visual pathway which terminates in the lateral geniculate nucleus of the thalamus (LGN). The LGN is a ‘relay’ nucleus of the dorsal thalamus which projects via the optic radiation to the primary visual cortex at the occipital pole. Visual cortex

Optic nerve Optic chiasm Optic tract Lateral Geniculate Nucleus Whole brain ventral surface Optic nerve Optic chiasm Optic tract Lateral Geniculate Nucleus Optic radiation We focus here on the primary visual pathway by looking at the ventral surface of this whole brain, the optic nerve, the second cranial nerve passes from the eye to the base of the anterior hypothalamus, just anterior to the infindibulum; the nerves from each eye meet and form the optic chiasm. At this point a partial decussation of the optic axons takes place. Axons from the nasal half of the eye cross the midline and axons from the temporal half retina remain uncrossed. Why such a crossing pattern will become evident in a moment. (As we will see in more detail shortly the purpose of the partial decussation is to bring together corresponding points in the visual field. Post chiasm the continuation of the optic axons is called the optic tract.) The optic tract passes caudally along the lateral surface of the diencephalon to terminate in a large thalamic nucleus called the lateral geniculate nucleus (LGN). The LGN is the thalamic relay for visual signals to primary visual cortex. Output of the LGN travels via the optic radiation - in the internal capsule around the lateral wall of the lateral ventricle - to reach the cortex in the occipital pole that forms the walls of the calcarine sulcus.

How do these fields map onto primary visual cortex How do these fields map onto primary visual cortex. First we need to know the layout of cortex. The primary visual cortex is buried deep on the banks of the calcarine sulcus which lies on the medial surface of the occipital lobe. In this figure the calcarine sulcus is indicated by the yellow line. A small portion of visual cotex, representing the upper and lower vetical meridians protrudes out onto the lips of the sulcus, indicated by the yellow hatching. In the next slide we will unfold the primary visual cortex and look at the way the visual hemifield is represented on the cortical surface.

Lesions of Visual Pathway

Lesions of Visual Pathway V1 V1 LGN LGN Optic Chiasm bitemporal hemianopaia

Q1 Which of the following NOT feature of cons receptor A) about6 millions / retina B)Found at At Center (Fovea centralis) C) has Photopsin (3types). D)More sensitive to light.

Q2 Transduction of dark vision inhibited by A)glutamate releasing B)Na influx C)Bipolar cell block D) Hyperoplrization

Q3 Which of the following DOSE NOT considered as visual pathway Station A)Optic disc B)Optic tract C)Medial geniculate Body of the Thalamus D)Visual cortex

Q4 Decassation of nasal nerve occur in: A)Optic disc B)Optic chisma C)Thalamus D)Visual cortex

References Human physiology by Lauralee Sherwood, 7th edition Text book physiology by Guyton &Hall,12th edition Text book of physiology by Linda .s contanzo,third edition